well we cant assume that because we cant go in the past... BUT, then again we cant assume gravity exists then, because there could be foriegn variables that are at play that we dont know about, but we can pretty much say it does exist because it fits physics as we know it.

as long as we dont know everything then anything can be different then what it seems, but some theorys are worth take that risk since we are about 99.9% were right on it, your decisions are based on connections in the brain. WE THINK, we could be wrong but its assumable.